The present invention relates to improvements in grinding apparatus or attrition mill, which include a pair of facially opposed, axially adjustable and relatively rotating grinding members defining therebetween a grinding space into which the raw material is passed, and during which passage substantial moments of axial thrust forces are generated opposing the means provided for maintaining the desired grinding clearance between the grinding members.
The invention relates more particularly to a rotating disc type grinding apparatus for refining paper pulp and the like usages, in which the pulp material to be ground or otherwise treated is passed into a grinding space defined between a pair of relatively axially adjustable grinding discs which rotate relative to one another in a plane perpendicular to their axes. At least one of the pair of discs is displaceable axially and is mounted on a rotating shaft which is free to move axially with the displaceable grinding member in response to pressure forces acting thereon. The pulp material, which may consist of wood chips, bagasse, fiber suspensions or similar material, is fed into the central portion of the grinding space, through which it is radially accelerated by the effect of the centrifugal force generated by the rotary movement of the discs. The resultant grist is ejected from the grinding space upon completion of the grinding operation, through a peripheral gap between the discs into a surrounding housing.
The axial movement or "float" of the rotating shaft is controlled to maintain the predetermined grinding clearance ranges between the discs, which clearance varies, depending on the particular application of the grinding apparatus. For instance, in conventional pulp refiners, the usual disc separation is between 0.1 mm. and 1 mm., whereas, in the application of the apparatus to waste paper (asphalt dispersion), the separation may be as much as 2.5 mm. In other applications, the discs may be spaced apart as little as 0.05 mm.
Pulp refining apparatus of the type described are generally exemplified by my U.S. Pat. Nos. 4,082,233, 4,253,233, 4,283,016 and 4,378,092.
The rapid acceleration of the material through the narrow grinding space generates axial thrust forces which tend to urge the discs away from one another and thus widen the grinding clearance, with consequent severe impairment of the efficiency of the apparatus.
If the grinding apparatus or attrition mill is operated as part of a closed and pressurized system for treating a fluid slurry, for example, in addition to the axial thrust forces acting on the discs, additional power must be imparted to the driving means, not only to drive the discs so as to achieve the desired attrition or grinding work, but also to drive the discs against the fluid friction or hydraulic drag forces acting on them, thus further adding to the axial load variations on the rotating shaft.
It should be understood that, unless these forces are effectively counteracted, the apparatus would break down or be rendered useless.
It should also be understood that the resistance to these thrust forces increases tremendously as the diameter of the discs increases.
Because of the growing demand for large capacity refining systems, which call for large diameter grinding discs, such as on the order of 150 cm. or larger, the absorption of these axial thrust forces has become an increasingly accentuated problem.
Late developments involve refiners having a diameter of 165 cm.-170 cm., with a rotational speed of 1500 r.p.m.-3600 r.p.m., capable of a power input of 15,000 kw.-40,000 kw.
For a better understanding of the tremendous axial loads or thrust forces imposed on the rotating shaft, let us assume that a 150 cm. diameter disc rotating at 1800 r.p.m. will generate a centrifugal force corresponding to about 2800 g's accelerating the grist through the grinding space, which centrifugal force will impose an axial load on the shaft of about 100 tons, to be absorbed by the bearing construction. Now, if the speed of the grinding disc is doubled, i.e. increased to 3600 r.p.m. the centrifugal force will be increased by a factor of 4, according to Newton's law of force and motion. Thus, the centrifugal force will be increased to 11,200 g's, which might increase the axial load on the rotating shaft to the order of 200-300 tons. These abnormally heavy axial loads have to be distributed over a complicated bearing system requiring a multiplicity of bearings and servo motors, with consequent increase in dimensions and cost of manufacture of the apparatus.
An example of a bearing construction of the above mentioned type is disclosed in my U.S. Pat. No. 3,717,308, issued Feb. 20, 1973, on an application originally filed July 5, 1969. This patent discloses and claims a bearing system with combined axial and radial thrust bearings supporting the rotating shaft, each bearing being connected to a servo motor for absorbing the axial thrust forces imposed upon the rotating shaft. Other examples of bearing constructions heretofore used are disclosed in my U.S. Pat. No. 4,118,800, issued Oct. 3, 1978, U.S. Pat. No. 3,212,721 to Asplund et al, issued Oct. 19, 1965, U.S. Pat. No. 4,073,442 to Nils G. Virving, dated Feb. 14, 1978, and U.S. Pat. No. 3,276,701, issued to Sprout Waldron & Co., Inc., assignee of Chester Donald Fisher, dated Oct. 4, 1966.
U.S. Pat. No. 4,402,463, issued Sept. 6, 1983, to Escher Wyss GmbH, assignee of Albrecht Kahmann et al, suggests another solution of the problem discussed herein.
Common to the prior art references is the fact that the hydraulic pistons in the servo motors for the thrust bearings are non-rotating.